Communications in an industrial application includes, for example, communications in process control. Process control is made by a distributed control system. A distributed control system is used for plant operation control in a wide range of fields such as petroleum chemistry, steel, paper pulp, and electric power. Communications in an industrial application will be described by taking as an example a distributed control system.
FIG. 1 shows an exemplary configuration of a general distributed control system.
In FIG. 1, an operation monitoring apparatus 1 and a controller 2 are connected to a control bus 3. The controller 2 controls a plant 4 under the monitoring of the operation monitoring apparatus 1. The operation monitoring apparatus 1 is in charge of plant operation and monitoring. The operation monitoring apparatus 1 displays a screen for control operation and monitoring. In accordance with the scale of a plant, a plurality of controllers is distributed in the plant. The operation monitoring apparatus 1 and the controller 2 communicates with each other via a ¥ the control bus 3 to control the plant.
Sensor devices 5, 6 in the plant 4 detect process values of temperature, pressure, liquid level and the like. Degrees of openings of valves 7, 8 are controlled by way of an operation signal provided from the controller 2. An analog signal of 1 to 20 mA and 1 to 5V output from the sensor devices 5, 6 is input to the controller 2. Based on this input, the control unit (not shown) in the controller 2 performs control arithmetic operation and obtains an operation quantity. The operation quantity is output as an analog signal of 1 to 20 mA and 1 to 5V. This output is used to control the degree of opening of the valves 7, 8. For example, by controlling the degree of valve opening on a reaction oven, the process quantities of temperature and pressure are controlled.
A control bus in a conventional distributed control system is dedicated to process control. The protocol used by the control bus has been dedicated to process control.
Recently, with the remarkable progress in the IT (Information Technology) and web-related technologies, an open architecture has been demanded of the control bus of a distributed control system. With this background, it is under examination to apply a general-purpose bus and a standard protocol to a control bus. In order to apply a general-purpose bus and a standard protocol to a control bus, it is necessary to satisfy the requirements of communications in an industrial application.
Patent document 1 describes a communication system that determines whether an identifier attached to receive data takes priority and performs priority control of communications in accordance with the determination result thus assuring the real time properties of communications of priority data.
Patent document 1: Japanese patent No. 3457636
Conventionally, communications in an existing industrial application that is based on the Ethernet (registered trademark) and TCP/IP (Transmission Control Protocol/Internet Protocol) implements cyclic scan transmission on the UDP service in order to provide real time communications. This communication system (conventional communication system) assures some degree of real time properties although it does not sufficiently satisfy the scalability and flexibility. This is described below.
In order to perform data exchange in a large-scale system such as a distributed control system, use of a conventional communication system requires a large-capacity storage area.
Since a distributed control system performs broadcast communications, exchange of large amount of data may render a receiving communication band insufficient in conventional communication system. Thus, the conventional communication system is not advantageous in application to a large-scale system or exchange of large amount of data and does not sufficiently meet the need of industrial applications.
In the conventional communication system, data exchanged between communication stations is fixed. For example, given communication stations A, B, C and D, communication station A is set to exchange data with communication stations B, C and D. Communication station A is not designed to exchange data with communication station C alone. Communication station A must change its setting in order to exchange data with communication station C alone.
From this, it is not possible to selectively acquire necessary information from large amount of information on a network. Even when this is possible, the user needs time and workload for changing the setting.
Thus, the conventional communication system does not sufficiently meet the needs of industrial applications in terms of flexibility.
The invention has been accomplished to solve the problems. An object of the invention is to provide a communication control system that meets the needs of industrial applications in terms of real time properties as well as scalability and flexibility by implementing a real time communication protocol using a UDP service on the Ethernet (registered trademark) and a standard communication protocol such as UDP/IP.